David,
In what way is it heavyweight? I generally use xscreensaver, which runs as a daemon; 'xscreensaver-command -lock' locks the screen. Pretty graphicy things run as separate processes, so I think starting it up should be pretty cheap.
My debian system has about 12'000 files installed already. Perhaps it's just me, but installing another 40+ files for something as simple as "lock my computer until I got coffee" is not what I was looking for when I switched to Linux. But as Gregory pointed out, xtrlock *is* a perfect alternative for me ... and the executable is only about 6kb. :)
Kind regards, Remo